Second battery and separator used therein

ABSTRACT

An object of the present invention is to provide a secondary battery that is able to inhibit the growth of a dendrite that can generate from an electrode comprising alkali metal and a separator used therein. 
     A secondary battery, comprising: 
     a positive electrode; 
     a negative electrode comprising alkali metal; 
     a separator comprising a layer of tetrafluoroethylene (TFE) polymer or copolymer that reacts with a dendrite of the alkali metal, the separator being hydrophilized at a rate of not less than 10% and not more than 80%; and 
     a layer that does not react with a dendrite of the alkali metal located between the separator and the negative electrode, and a separator used therein.

TECHNICAL FIELD

The present invention relates to a secondary battery and a separatorused therein. More particularly, the present invention relates to asecondary battery that is able to inhibit the growth of a dendrite thatcan generate from an electrode comprising alkali metal and a separatorused therein.

BACKGROUND ART

In recent years, portable cordless products such as a CD player,multimedia player, cellular phone, smartphone, notebook personalcomputer, tablet device, and video camera have been increasinglyminiaturized and made portable. Further, from the standpoint ofenvironmental issues such as air pollution and increased carbon dioxide,hybrid vehicles and electric vehicles have been developed and are at thestage of practical use. Such electronics and electric vehicles requirean excellent secondary battery having characteristics such as highefficiency, high output, high energy density, and light weight. As asecondary battery having such properties, various secondary batterieshave been developed and researched.

A chargeable and dischargeable secondary battery generally has astructure that prevents direct electrical contact between a positiveelectrode and a negative electrode by separating the positive electrode(cathode) and the negative electrode (anode) with a porous polymermembrane comprising an organic electrolyte solution.

Until now, V₂O₅, Cr₂O₅, MnO₂, TiS₂, and the like are known as a positiveelectrode active material of this nonaqueous electrolyte secondarybattery. In addition, in lithium ion batteries that are currentlycommercialized, LiCoO₂, LiMn₂O₄, LiNiO₂, and the like are used as a 4-Vclass positive electrode active material.

On the other hand, as a negative electrode, alkali metals includingmetallic lithium have been studied so much. This is because, inparticular, metallic lithium has a very high theoretical energy density(3861 mAh/g by weight capacity density) and a low charge/dischargepotential (−3.045 V vs. SHE) and thus is considered to be an idealnegative electrode material.

Then, as an electrolyte solution, for example, a lithium salt dissolvedin a nonaqueous organic solvent is used, which salt has good ionicconductivity and negligible electrical conductivity. During charging,lithium ions move from the positive electrode to the negative electrode(lithium). During discharging, the lithium ions move in the reversedirection back to the positive electrode.

However, using lithium metal as a negative electrode has the followingproblem. Dendritic lithium (lithium dendrite) precipitates on thelithium surface of the negative electrode during charging. The dendriticlithium grows as the charge and discharge is repeated, causing, forexample, detachment from the lithium metal to thereby reduce cyclecharacteristics. In the worst case, the dendritic lithium grows to theextent that it breaks through the separator, causing a short circuit ofa battery, which can cause firing of the battery.

Thus, to use lithium metal as a negative electrode, the problem oflithium dendrite needs to be solved.

Thus, various carbonaceous materials, metals such as aluminum, alloys oroxides thereof, and the like that are able to occlude and releaselithium have been studied so much.

However, using these negative electrode materials reduces the capacityas a battery while it is effective for inhibiting the growth of alithium dendrite.

Consequently, the research and development for using metallic lithium asa negative electrode has still been actively conducted, and a number ofimprovements such as development of an electrolyte solution and study ofa battery-constituting method have been made.

For example, Patent Document 1 (JP 05-258741 A) proposes using aseparator with a smaller pore size than that of conventional ones sothat a crystal that grows from the negative electrode side does not growat pore portions in order to inhibit the growth of such a dendriticcrystal (dendrite).

Also, Patent Document 2 (JP 09-293492 A) proposes using an expandedporous polytetrafluoroethylene (PTFE) membrane as a battery separatorwith high porosity, mechanical strength, and heat resistance andtreating the surface and internal pore surface of the expanded porousPTFE membrane to modify these surfaces to a hydrocarbon or carbon oxidecompound and cover them. This is because lithium metal reacts with PTFE.Namely, the separator (PTFE) is in contact with the whole surface of anegative electrode (lithium), and consequently a reaction occurs at theelectrode/separator interface, whereby the lithium electrode surface iscovered with a reaction product, which adversely affects theelectrolysis/precipitation of the lithium. To solve this problem, PatentDocument 2 describes that the reaction between lithium and a PTFEsubstrate can be prevented by treating the surface and internal poresurface of the expanded porous PTFE membrane to modify these surfaces toa hydrocarbon or carbon oxide compound and cover them.

Further, Patent Document 3 (U.S. Pat. No. 5,427,872) discloses a lithiumelectrode (anode) secondary battery comprising a first porous separatorand a second separator, wherein the first separator is adjacent to theanode and formed by an aliphatic hydrocarbon resin that does not reactwith lithium and lithium ions, and the second separator is locatedbetween the first separator and the cathode and comprises thermoplasticpolytetrafluoroethylene that reacts with lithium metal. There isdescribed that, in this secondary battery, when the tip of a lithiumdendrite grows from the anode surface and penetrates the first separatorto touch the second separator, the tip of the dendrite and thethermoplastic polytetrafluoroethylene of the second separator causes anexothermic reaction, and the thermoplastic polytetrafluoroethylenedissolves to form non-porous blocked parts, which prevents the dendritefrom further growing.

However, a means of more reliably inhibiting the growth of a dendrite isstill demanded.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 05-258741 A

Patent Document 2: JP 09-293492 A

Patent Document 3: U.S. Pat. No. 5,427,872

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Patent Document 1 to Patent Document 3 described above are all forachieving a secondary battery using lithium metal.

However, the separator with a small pore size of Patent Document 1cannot completely prevent the growth of a lithium dendrite in principleas long as ions pass therethrough to cause precipitation despite thesmall pores.

According to Patent Document 2, although the reaction between theelectrode (lithium)/the separator (fluorine content) can be prevented,the growth of a lithium dendrite cannot be prevented.

The heating value (theoretical value) of the reaction of a lithiumdendrite and polytetrafluoroethylene described in Patent Document 3 isnot sufficient to dissolve polytetrafluoroethylene and form non-porousblocks. Actually, retests were carried out, but polytetrafluoroethylenecould not be dissolved to form non-porous blocks by this reaction.

In any of the cases, there remains a possibility that a final shortcircuit between a lithium dendrite and a positive electrode or reducedcycle characteristics due to a detached and isolated lithium dendriteoccurs.

Further, in any of Patent Document 1 to Patent Document 3, ahydrophilizing treatment of the separator is neither disclosed norsuggested. Therefore, it can be thought that a hydrophilizing treatmenthas not been performed in Patent Document 1 to Patent Document 3. Anunhydrophilized separator does not provide sufficient batteryperformance because it does not get wet sufficiently by an electrolytesolution and ions cannot move smoothly.

If by any chance a hydrophilizing treatment of the separator isperformed, unless there is a particular reason, it is evident that theseparator is completely hydrophilized so that ions can move smoothly andthe original battery performance can be obtained. In Patent Documents 1to 3, such a particular reason is neither disclosed nor suggested. Inthis case, the separator comprising PTFE in Patent Documents 1 to 3 iscompletely hydrophilized, i.e., completely covered with a hydrophilizingtreatment material, and PTFE (separator) and a lithium dendrite will notcause a reaction.

This means that the invention of Patent Document 3 (PTFE is dissolved byheat of reaction between PTFE and lithium to form blocks) is not valid.In other words, it is considered that a hydrophilizing treatment has notbeen carried out in the invention of Patent Document 3, and thereforesufficient battery performance cannot be obtained.

Further, Patent Document 2 is also on the assumption that PTFE reactswith lithium, and if hydrophilization is complete, the problem of PatentDocument 2 does not exist in the first place. In other words, it isconsidered that a hydrophilizing treatment has not been carried out alsoin the invention of Patent Document 2, and therefore sufficient batteryperformance cannot be obtained.

Patent Document 1, as mentioned above, can not completely prevent thegrowth of a lithium dendrite in principle. Therefore, if ahydrophilizing treatment is carried out to make PTFE unreactive withlithium, the growth of a dendrite is rather promoted, and it is morelikely that the dendrite reaches the cathode to cause a short circuit.

Although the examples using lithium metal were explained in the above,it is known that alkali metals other than lithium also have a very hightheoretical energy density and a low charge/discharge potential and cangenerate a dendrite.

Thus, an object of the present invention is to provide a secondarybattery that is certainly able to inhibit the growth of a dendrite thatcan generate from an electrode comprising alkali metal and a separatorused therein.

Means for Solving the Problems

The present invention provides the following aspects.

(1)

A secondary battery, comprising:

-   a positive electrode;-   a negative electrode comprising alkali metal;-   a separator comprising a layer of tetrafluoroethylene (TFE) polymer    or copolymer that reacts with a dendrite of the alkali metal, the    separator being hydrophilized at a rate of not less than 10% and not    more than 80%; and-   a layer that does not react with a dendrite of the alkali metal    located between the separator and the negative electrode.    (2)

The secondary battery according to (1), wherein the layer that does notreact with a dendrite of the alkali metal is a part of the separator,and, in the layer that does not react with a dendrite of the alkalimetal, the inner surface of its pores is at least partially covered witha material that does not react with a dendrite of the alkali metal.

(3)

The secondary battery according to (1), wherein the layer that does notreact with a dendrite of the alkali metal is independent of theseparator.

(4)

The secondary battery according to (3), wherein the layer that does notreact with a dendrite of the alkali metal comprises any one of glasscomprising SiO_(x) (0<x≦2), polyvinylidene fluoride (PVDF), polyimide(PI), polyethylene (PE), or polypropylene (PP) or a mixture thereof.

(5)

The secondary battery according to (3), wherein the layer that does notreact with a dendrite of the alkali metal comprises any one of inorganicoxides selected from the group consisting of alumina, titanium oxide,sodium oxide, calcium oxide, boron oxide, potassium oxide, and leadoxide or a mixture thereof and a binder.

(6)

The secondary battery according to any one of (1) to (5), wherein in thelayer that does not react with a dendrite of the alkali metal, the innersurface of its pores is covered with a material other thantetrafluoroethylene (TFE) polymer or copolymer.

(7)

The secondary battery according to (6), wherein the material other thantetrafluoroethylene (TFE) polymer or copolymer is any one of glasscomprising SiO_(x) (0<x≦2), polyvinylidene fluoride (PVDF), polyimide(PI), polyethylene (PE), or polypropylene (PP) or a mixture thereof.

(8)

The secondary battery according to any one of (1) to (7), wherein thelayer that does not react with a dendrite of the alkali metal ishydrophilized.

(9)

The secondary battery according to any one of (1) to (8), wherein thetetrafluoroethylene (TFE) polymer or copolymer is expanded or expandedporous.

(10)

The secondary battery according to any one of (1) to (9), wherein thetetrafluoroethylene (TFE) polymer or copolymer is expandedpolytetrafluoroethylene, perfluoro alkoxy alkane (PFA),tetrafluoroethylene/hexafluoropropene copolymer (FEP),ethylene/tetrafluoroethylene copolymer (ETFE), orethylene/chlorotrifluoroethylene copolymer (ECTFE) or a mixture thereof.

(11)

The secondary battery according to any one of (1) to (10), wherein thethickness of the layer that does not react with a dendrite of the alkalimetal is 0.1 μm or more.

(12)

The secondary battery according to any one of (1) to (11), wherein theseparator at least comprises fluorine that can react with the total massof the alkali metal constituting the negative electrode.

(13)

The secondary battery according to any one of (1) to (12), wherein thealkali metal is lithium or sodium.

(14)

The secondary battery according to any one of (1) to (13), wherein thesecondary battery further comprises a shut-down layer.

(15)

The secondary battery according to (14), wherein the shut-down layer islocated between the separator and the positive electrode.

(16)

The separator used in the secondary battery according to any one of (1)to (15).

Effects of the Invention

The present invention provides a secondary battery that is certainlyable to inhibit the growth of a dendrite that can generate from anelectrode comprising alkali metal and a separator used therein.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a coin cell.

BEST MODE FOR CARRYING OUT THE INVENTION

The secondary battery of the present invention is characterized bycomprising the following:

-   a positive electrode;-   a negative electrode comprising alkali metal;-   a separator comprising a layer of tetrafluoroethylene (TFE) polymer    or copolymer that reacts with a dendrite of the alkali metal, the    separator being hydrophilized at a rate of not less than 10% and not    more than 80%; and-   a layer that does not react with a dendrite of the alkali metal    located between the separator and the negative electrode.

Secondary batteries are basically composed of a positiveelectrode/negative electrode and a separator comprising an electrolytethat acts as an ion-conducting medium between the two electrodes.

The negative electrode comprises alkali metal. Alkali metals have a veryhigh theoretical energy density and a low charge/discharge potential andthus are considered to be an ideal negative electrode material. Inparticular, among the alkali metals, lithium has a very high theoreticalenergy density (3861 mAh/g by weight capacity density) and a lowcharge/discharge potential (−3.045 V vs. SHE) and thus is considered tobe an ideal negative electrode material.

During charging, ions in the separator move from the positive electrodeto the negative electrode. During discharging, the ions move in thereverse direction back to the positive electrode.

During charging, dendritic alkali metal (dendrite) precipitates on thesurface of the negative electrode comprising alkali metal. The dendritegrows as the charge and discharge is repeated, causing, for example,detachment from the negative electrode metal to thereby reduce cyclecharacteristics. In the worst case, the dendrite grows to the extentthat it breaks through the separator, causing a short circuit of abattery, which can cause firing of the battery.

The separator serves to prevent a short circuit by separating thepositive electrode and the negative electrode and to ensure high ionicconductivity by retaining an electrolyte necessary for cell reaction.The separator comprises a layer of tetrafluoroethylene (TFE) polymer orcopolymer. This is because the tetrafluoroethylene (TFE) polymer orcopolymer has high porosity, high strength, and excellent heatresistance. This tetrafluoroethylene (TFE) polymer or copolymer containsfluorine. This fluorine is known to react with an alkali metal dendriteaccording to the following formula:—[CF₂—CF₂]—_(n)+4nA→=[C═C]=_(n)+4nAF

wherein A means alkali metal.

It has been believed that, in tetrafluoroethylene (TFE) polymer orcopolymer, once the fluorine contained reacts with alkali metal,defluoridation (i.e., carbonization) occurs, and high porosity, highstrength, and heat resistance cannot be maintained. On the contrary, thepresent inventor arrived at the idea of taking advantage of thecharacteristics of this fluorine to react with alkali metal. Namely, thepresent invention has been completed based on a novel idea of thepresent inventor that the growth of a dendrite can be inhibited byreacting fluorine with the dendrite of alkali metal.

The tetrafluoroethylene (TFE) polymer or copolymer constituting theseparator is fluororesin and hydrophobic in itself. However, theseparator must be those in which ions existing in an electrolytesolution (aqueous solution, organic solvent, and the like) are able topass through a porous body or fibers of the separator and move from oneplace to the other place separated by the separator. Accordingly, thetetrafluoroethylene (TFE) polymer or copolymer constituting theseparator is subjected to a hydrophilizing treatment. The hydrophilizingtreatment must be carried out sufficiently such that the separator hashydrophilicity and that the interior of the separator is made wet withan electrolyte solution. However, in the present invention, carrying outthe hydrophilizing treatment at a rate of not less than 10% and not morethan 80% is one of the characteristics. Thus, at least a part of theseparator shall not be hydrophilized, whereby the fluorine content thatis contained in tetrafluoroethylene (TFE) constituting the separator andreacts with a dendrite will remain exposed. In this case, since thefluorine content that reacts with a dendrite remains exposed, thisfluorine content certainly reacts with a dendrite to inhibit the growthof the dendrite. When the rate of the hydrophilizing treatment is lessthan 10%, the hydrophilicity is not sufficient, i.e., the ionicconductivity is not sufficient. In this case, a battery will have a highinternal resistance, and the original battery performance cannot beobtained. On the other hand, when the rate of the hydrophilizingtreatment is more than 80%, the fluorine content that reacts with analkali metal dendrite is not sufficiently exposed, and the dendritegrowth-inhibiting effect decreases.

Carrying out this hydrophilizing treatment at a rate of not less than10% and not more than 80% can be controlled as appropriate by the methodof hydrophilizing treatment mentioned below.

The method of hydrophilizing treatment is not particularly limited, andthe method described in Japanese Patent No. 3463081, which is patentedby the present applicant, may be used. The method, in summary, is amethod in which a gelled product in the form of a solution formed bypartial gelation reaction of a hydrolyzable metal-containing organiccompound (for example, silicone alkoxide such as tetraethoxysilane) isattached to at least the microfibrils/micronodes or pore wall surface ofa polymeric porous body having continuous pores to complete thegelation, thereby providing a structure of being covered with a metaloxide gel formed as a result of drying. For example, when siliconealkoxide such as tetraethoxysilane is used as a hydrolyzablemetal-containing organic compound, hydrophilization can be achieved bycovering with silica gel.

Alternatively, a hydrophilic polymer (for example, PVA or the like) maybe impregnated into a porous body, and then dried for formation toprovide a structure of being covered with the hydrophilic polymer.

The state of hydrophilization can be measured by various surfaceanalysis methods, and, for example, tetrafluoroethylene (TFE) that hasbeen subjected to a hydrophilizing treatment can be measured using aField Emission-Scanning Electron Microscope (FE-SEM for short). Thestate of a porous structure can be confirmed on an electron micrograph.For example, the state of TFE having a hydrophilized layer of several nmto several tens of nm on its surface of nodes and fibrils whilemaintaining the porous structure can be confirmed. Further, the ratio ofelements present on a sample surface can be measured using a compositionanalysis function of the electron microscope. Specifically, in the caseof TFE (C₂F₄) before hydrophilizing treatment, the abundance ratio F andC is F/C=2:1(66.7%:33.3%). Here, if SiOx is used as a hydrophilizingtreatment material to cover TFE, Si and O are present on the samplesurface. The rate of hydrophilization (the coverage by thehydrophilizing treatment material) can be determined from the ratio of Fpresent on the surface after hydrophilizing treatment.

Further, the secondary battery of the present invention comprises alayer that does not react with a dendrite of the alkali metal betweenthe negative electrode comprising alkali metal and the separatorcomprising a layer of tetrafluoroethylene (TFE) polymer or copolymerthat reacts with a dendrite of the alkali metal. Otherwise, i.e., if thenegative electrode comprising alkali metal and the separator that reactswith a dendrite of the alkali metal are brought into direct contact, thefluorine content in the separator (comprising the layer oftetrafluoroethylene (TFE) polymer or copolymer) reacts with the alkalimetal of the negative electrode on the whole contacting surface, anddefluoridation of the separator proceeds with or without the occurrenceof a dendrite; as a result, high porosity, high strength, and heatresistance cannot be maintained. Namely, the function as a separatorcannot be performed.

By inserting the layer that does not react with a dendrite between thenegative electrode and the separator, direct contact between theseparator that reacts with a dendrite and the negative electrode can beavoided.

The layer that does not react with a dendrite as well as the separatorserves to prevent a short circuit by separating the positive electrodeand the negative electrode and to ensure high ionic conductivity byretaining an electrolyte necessary for cell reaction. Thus, a materialhaving high porosity, high strength, and excellent heat resistance isused. Therefore, a dendrite that starts to grow from the negativeelectrode grows through a hole in the layer that does not react with adendrite. Since the layer that does not react with a dendrite does notreact with a dendrite, its hole structure is soundly maintained evenwhen a dendrite has grown. A dendrite passes through the hole in thelayer that does not react with a dendrite and finally reaches theseparator. Since the separator comprises a layer of tetrafluoroethylene(TFE) polymer or copolymer, the fluorine content contained in theseparator reacts with a dendrite of the alkali metal, and the growth ofa dendrite stops here. The time when and the place where a dendritereaches the separator vary depending on the path of the hole in thelayer that does not react with a dendrite, and reaction between adendrite and the fluorine content in the separator occurs dispersedly interms of time and place. Therefore, the situation where the function asa separator cannot be performed because of defluoridation, i.e.,carbonization due to temporal and local reaction of the fluorine contentin the separator with a dendrite is significantly prevented. This alsosolves the problem in that a dendrite penetrates the separator to causea short circuit between the negative electrode and the positiveelectrode.

As the positive electrode constituting the battery, any conventionalmaterial known or well-known as a positive electrode for a lithiumsecondary battery can be used.

Although the material used as a positive electrode in the secondarybattery of the present invention is not particularly limited, metalchalcogenides, which are able to occlude and release alkali metal ionssuch as sodium ions and lithium ions during charging and discharging,and the like are preferred. Examples of such metal chalcogenides includeoxides of vanadium, sulfides of vanadium, oxides of molybdenum, sulfidesof molybdenum, oxides of manganese, oxides of chromium, oxides oftitanium, sulfides of titanium, and complex oxides and complex sulfidesthereof. Examples of such compounds include Cr₃O₈, V₂O₅, V₅O₁₈, VO₂,Cr₂O₅, MnO₂, TiO₂, MoV₂O₈, TiS₂V₂S₅MoS₂, MoS₃VS₂, Cr_(0.25)V_(0.75)S₂,Cr_(0.5)V_(0.5)S₂ and the like. Further, LiMY₂ (M is a transition metalsuch as Co and Ni, and Y is a chalcogenide such as O and S), LiM₂Y₄ (Mis Mn, and Y is O), oxides such as WO₃, sulfides such as CuS,Fe_(0.25)V_(0.75)S₂, and Na_(0.1)CrS₂, phosphorus-sulfur compounds suchas NiPS₈ and FePS₈, selenium compounds such as VSe₂ and NbSe₃, ironcompounds such as iron oxides, and the like can also be used. Further,manganese oxides and lithium/manganese complex oxide having a spinelstructure are also preferred.

More specific examples of the material include LiCoO₂,LiCo_(1-x)Al_(x)O₂, LiCo_(1-x)Mg_(x)O₂, LiCo_(1-x)Zr_(x)O₂, LiMn₂O₄,Li_(1-x)Mn_(2-x)O₄, LiCr_(x)Mn_(2-x)O₄, LiFe_(x)Mn_(2-x)O₄,LiCo_(x)Mn_(2-x)O₄, LiCu_(x)Mn_(2-x)O₄, LiAl_(x)Mn_(2-x)O₄, LiNiO₂,LiNi_(x)Mn_(2-x)O₄, Li₆FeO₄, NaNi_(1-x)Fe_(x)O₂, NaNi_(1-x)Ti_(x)O₂,FeMoO₄Cl, LiFe₅O₈, FePS₃, FeOCl, FeS₂, Fe₂O₃, Fe₃O₄, β-FeOOH, α-FeOOH,γ-FeOOH, α-LiFeO₂, α-NaFeO₂, LiFe₂(MoO₄)₃, LiFe₂(WO₄)₃, LiFe₂(SO₄)₃,Li₃Fe₂(PO₄)₃, Li₃Fe₂(AsO₄)₃, Li₃V₂(AsO₄)₃, Li₃FeV(AsO₄)₃,Li_(1.5)Al_(0.5)Ge_(1.5)(PO₄)₃, LiFePO₄, Li₂FeSiO₄, FeBO₃, FeF₃, and thelike.

In the separator between the positive electrode and the negativeelectrode, an electrolyte solution is retained. As the electrolytesolution, for example, an alkali metal salt such as a sodium salt or alithium salt dissolved in a nonaqueous organic solvent is used. Theelectrolyte solution is not particularly limited as long as it has goodionic conductivity and negligible electrical conductivity, and anyconventional material known or well-known as an electrolyte solution fora lithium secondary battery can be used.

Examples of nonaqueous solvents that can be used for the electrolytesolution of the secondary battery of the present invention includeacetonitrile (AN), γ-butyrolactone (BL), γ-valerolactone (VL),γ-octanoic lactone (OL), diethyl ether (DEE), 1,2-dimethoxyethane (DME),1,2-diethoxyethane (DEE), dimethyl sulfoxide (DMSO), 1,3-dioxolane(DOL), ethylene carbonate (EC), propylene carbonate (PC), dimethylcarbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC),methyl formate (MF), tetrahydrofuran (THF), 2-methyltetrahydrofuran(MTHF), 3-methyl-1,3-oxaziridin-2-one (MOX), sulfolane (S), and thelike, which can be used alone or as a mixture of two or more thereof.

Examples of the alkali metal salt, particularly, the lithium salt usedfor the electrolyte solution of the secondary battery include lithiumsalts such as LiPF₆, LiAsF₆, LiClO₄, LiBF₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, andLiC₄F₉SO₃, and one, or two or more of them are dissolved in thenonaqueous solvent described above at a concentration of about 0.5 to2.0 M to obtain a nonaqueous electrolyte solution.

In one aspect of the present invention, the layer that does not reactwith a dendrite of the alkali metal may be a part of the separator, and,in the layer that does not react with a dendrite of the alkali metal,the inner surface of its pores may be at least partially covered with amaterial that does not react with a dendrite.

In this aspect, in a part of the separator, the inner surface of itspores is at least partially covered with a material that does not reactwith a dendrite, and this part is defined as a layer that does not reactwith a dendrite of the alkali metal. As mentioned above, the separatorcomprises a layer of tetrafluoroethylene (TFE) polymer or copolymer, andthe fluorine content contained in this layer reacts with a dendrite.However, in this aspect, since the layer of tetrafluoroethylene (TFE)polymer or copolymer, i.e., the fluorine content contained in this layeris covered with a material that does not react with a dendrite, areaction between the fluorine content and a dendrite does not occur inthis area, and this area can be a layer that does not react with adendrite of the alkali metal.

According to this aspect, since a part of the separator is only at leastpartially covered with a material that does not react with a dendrite,the number of parts of the battery is reduced, which is advantageous inassembling a secondary battery.

In another aspect, the layer that does not react with a dendrite of thealkali metal may be independent of the separator.

According to this aspect, the above-mentioned step of covering a part ofthe separator with a material that does not react with a dendrite is notnecessary.

The layer that is independent of the separator and does not react with adendrite of the alkali metal may comprise any one of glass comprisingSiO_(x) (0<x≦2), polyvinylidene fluoride (PVDF), polyimide (PI),polyethylene (PE), or polypropylene (PP) or a mixture thereof. Thesematerials do not cause a reaction with a dendrite of the alkali metal.In addition, these materials allow appropriate preparation of one havinghigh porosity, high strength, and heat resistance. Althoughpolyvinylidene fluoride (PVDF) contains fluorine content, a reactionbetween the fluorine content and a dendrite of the alkali metal will notoccur to progress defluoridation, i.e., carbonization. While not wishingto be bound by any particular theory, this is probably becausepolyvinylidene fluoride (PVDF) also contains hydrogen content anddefluoridation caused by direct reaction between the alkali metal andthe fluorine content is suppressed, or because hydrogen remains evenafter fluorine reacted with the alkali metal and carbonization issuppressed.

The layer that is independent of the separator and does not react with adendrite of the alkali metal may comprise any one of inorganic oxidesselected from the group consisting of alumina, titanium oxide, sodiumoxide, calcium oxide, boron oxide, potassium oxide, and lead oxide or amixture thereof and a binder. These materials also do not cause areaction with a dendrite of the alkali metal. In addition, thesematerials also allow appropriate preparation of one having highporosity, high strength, and heat resistance.

Regardless of whether the layer that does not react with a dendrite ofthe alkali metal is a part of the separator or independent of theseparator, the inner surface of its pores may be covered with a materialother than tetrafluoroethylene (TFE) polymer or copolymer.

The fluorine content contained in tetrafluoroethylene (TFE) cause areaction with a dendrite of the alkali metal. Materials other thantetrafluoroethylene (TFE) are less likely to react with a dendrite andable to further enhance the function of not reacting with a dendrite inthe layer that does not react with a dendrite has.

The method of covering a material is not particularly limited, and aconventional method can be appropriately used depending on the material.A material to be applied may be brought into solution for impregnation.For example, any method may be used such as vacuum pressureimpregnation, vacuum impregnation, spraying, evaporation to dryness,metering bar method, die coating method, gravure method, reverse rollmethod, doctor blade method, knife coating method, and bar coatingmethod.

Further, even when a solution is only applied to the inner surface ofpores, the solution fills a space. Namely, “impregnation” herein onlyrequires filling of a space of pores with a solution and is a conceptincluding application and the like.

The application method is not particularly limited, and, for example,any method such as metering bar method, die coating method, gravuremethod, reverse roll method, and doctor blade method may be used.

As a method of covering, chemical modification and physical modificationmay be used. Examples of the chemical modification include the method ofadding a functional group to the inner surface of pores by acetylation,isocyanation, acetalization, or the like, the method of covering theinner surface of pores with organic matter or inorganic matter bychemical reaction, and the like. Examples of the physical modificationinclude, physical vapor deposition such as vacuum deposition, ionplating, and sputtering, chemical vapor deposition, plating methods suchas electroless plating and electrolytic plating, and the like. Thesemethods for covering may be used alone, or two or more thereof may beused in combination.

When the layer that does not react with a dendrite is a part of theseparator, to enhance the binding force between a material to be appliedand a material to be covered (the raw material is tetrafluoroethylene(TFE)), if the inner surface of pores of the material to be covered(corresponding to the layer that does not react with a dendrite in asecondary battery) is subjected to a chemical treatment (alcoholsubstitution treatment, alkali treatment, or the like) or a physicaltreatment (corona treatment, plasma treatment, UV treatment, or thelike) as a pretreatment to attach a surface functional group to an F—Cbond at the inner surface of pores of the material to be covered, theadhesive strength will be enhanced.

The material other than tetrafluoroethylene (TFE) polymer or copolymerapplied to the inner surface of pores of the layer that does not reactwith a dendrite may be any one of glass comprising SiO_(x) (0<x≦2),polyvinylidene fluoride (PVDF), polyimide (PI), polyethylene (PE), orpolypropylene (PP) or a mixture thereof. These materials do not cause areaction with a dendrite of the alkali metal. In addition, thesematerials allow appropriate preparation of one having high strength andheat resistance. Further, these materials can be applied to the innersurface of pores of the layer that does not react with a dendrite asappropriate (without blocking the pores) and are also able to maintainthe high porosity of the layer that does not react with a dendrite asappropriate.

The layer that does not react with a dendrite of the alkali metal may behydrophilized.

The hydrophilization can provide the characteristic of not reacting witha dendrite of the alkali metal or improve the characteristic. This isprobably because a hydrophilic group or hydrophilic substance isattached to the inner surface of pores of the layer that does not reactwith a dendrite and this hydrophilic group or hydrophilic substance doesnot react with a dendrite.

The hydrophilizing treatment applied to the separator mentioned abovemay be applied to the layer that does not react with a dendrite. Even ifthe layer that does not react with a dendrite is a part of the separatorand contains the fluorine content that reacts with a dendrite, thefluorine content that reacts with a dendrite can be covered with ahydrophilic group or hydrophilic substance by progressing thehydrophilizing treatment of the inner surface of its pores, so that areaction with a dendrite will not occur there. In other words, thehydrophilizing treatment can provide a layer that does not react with adendrite.

The tetrafluoroethylene (TFE) polymer or copolymer constituting theseparator and the layer that does not react with a dendrite may beexpanded or expanded porous.

Expanded porous membranes of tetrafluoroethylene (TFE) polymer orcopolymer have been hitherto studied so much, and membranes with highporosity and high strength have been obtained. Tetrafluoroethylene (TFE)polymer or copolymer is known to have high crystallinity and have highstrength by itself. An expanded porous membrane of tetrafluoroethylene(TFE) polymer or copolymer is suitably obtained by expanding a precursorformed by melt fusion of fine powders of tetrafluoroethylene (TFE)polymer or copolymer (see each description of JP 56-45773 B, JP 56-17216B, and U.S. Pat. No. 4,187,390). By controlling the fusion conditions offine powders of tetrafluoroethylene (TFE) polymer or copolymer or theexpanding conditions of a precursor, a membrane with high porosity andhigh strength can be produced. In addition, tetrafluoroethylene (TFE)polymer or copolymer has a high melting point and is advantageous inthat it does not melt even at 250° C. or higher.

More specifically, an expanded porous membrane of tetrafluoroethylene(TFE) polymer or copolymer is obtained in such a manner that apaste-like formed body obtained by mixing fine powders oftetrafluoroethylene (TFE) polymer or copolymer with a forming assistantis expanded after removing or without removing the forming assistanttherefrom and optionally baked. In the case of uniaxial expanding,fibrils are oriented in the expanding direction, and, at the same time,a fibrous structure having holes between the fibrils is provided. In thecase of biaxial expanding, fibrils spread radially, and a web-likefibrous structure is provided in which a number of holes defined bynodes and fibrils are present.

The porosity can be controlled by expanding as appropriate. The porosityis not particularly restricted as long as the electrolyte solution canbe retained in the battery, and it may be preferably 30% or more, morepreferably 60% or more, and still more preferably 80% or more in orderto ensure impregnating ability and permeability. The porosity of theporous membrane can be calculated from an apparent density p measured inaccordance with the method for measuring apparent density defined in JISK 6885 using the following equation. (The following equation is fordetermining the porosity of PTFE as an example. Accordingly, the truedensity of PTFE is taken as 2.2. The value of the true density isadjusted depending on the material constituting the porous membrane.)Porosity (%)=[(2.2−ρ)/2.2]×100

The thickness of the porous membrane (the separator and the layer thatdoes not react with a dendrite) is not particularly restricted and maybe determined as appropriate depending on the application. In the caseof those which are arranged between the electrodes, it may be preferablyfrom 1 μm to 1000 μm. When the thickness is less than 1 μm, handling canbe difficult because of insufficient strength, and, on the other hand,when it is more than 1000 μm, it can be difficult to uniformlyimpregnate the electrolyte solution. The thickness of the porousmembrane arranged between the electrodes is more preferably from 10 μmto 500 μm and still more preferably from 20 μm to 200 μm.

This tetrafluoroethylene (TFE) polymer or copolymer is not particularlylimited as long as it has high porosity, high strength, and heatresistance and can react with a dendrite of the alkali metal. Morespecifically, the tetrafluoroethylene (TFE) polymer or copolymer may beexpanded polytetrafluoroethylene, perfluoro alkoxy alkane (PFA),tetrafluoroethylene/hexafluoropropene copolymer (FEP),ethylene/tetrafluoroethylene copolymer (ETFE), orethylene/chlorotrifluoroethylene copolymer (ECTFE) or a mixture thereof.

The thickness of the layer that does not react with a dendrite of thealkali metal may be 0.1 μm or more.

A dendrite starts to grow from the negative electrode, passes throughthe hole in the layer that does not react with a dendrite, and finallyreaches the separator. To disperse the time when and the place where adendrite reaches the separator so that the fluorine content in theseparator will not temporally and locally react with a dendrite to causedefluoridation, i.e., carbonization, it is preferable to adjust thelayer that does not react with a dendrite of the alkali metal to have anappropriate thickness. When the thickness is as described above, thetime when and the place where a dendrite reaches the separator will besufficiently dispersed. To further ensure the dispersion, the thicknessof the layer may be preferably not less than 1.0 μm and more preferablynot less than 10 μm. There is no particular upper limit on the thicknessof the layer, and the thickness can be set as appropriate from thestandpoint of reducing the space for the secondary battery.

The separator may at least comprise fluorine that can react with thetotal mass of the alkali metal constituting the negative electrode.

If by any chance all the alkali metals constituting the negativeelectrode react with the separator, the reaction of the alkali metalconstituting the negative electrode will complete in the separatorbecause the separator comprises fluorine that can react with the totalmass of the alkali metal constituting the negative electrode. Therefore,a short circuit from the negative electrode to the positive electrodedue to penetration of a dendrite through the separator can be certainlyprevented.

The alkali metal constituting the negative electrode may be lithium orsodium.

That is because metallic lithium has a very high theoretical energydensity (3861 mAh/g by weight capacity density) and a lowcharge/discharge potential (−3.045 V vs. SHE) and thus is considered tobe an ideal negative electrode material. Metallic sodium also has a hightheoretical energy density and a low charge/discharge potential.Although lithium or sodium has been reported to grow as a dendrite, thepresent invention can inhibit such growth of a dendrite.

The secondary battery according to the present invention may furthercomprise a shut-down layer.

The shut-down layer is a layer having a shut-down function. Theshut-down function is a function to break a current, in other words, afunction to suppress thermal runaway of a battery, when the temperatureof the battery increases. An example of the shut-down layer is notspecifically limited, provided that the shut-down layer is a layerhaving micro pores, which has a relatively low melting point so as toblock the pores when the temperature of the battery increases. Forexample, as the shut-down layer, a polyolefin, in particular, apolyethylene porous layer can be utilized. Also, it is not limited to amembrane, but may be a nanofiber web or fiber web. As another exampleother than the above-mentioned examples, the shut-down layer maycomprise a heat reactive small sphere and/or a PTC element.

The shut-down layer may be located between the separator and thepositive electrode.

The location of the shut-down layer is not specifically limited,provided that the shut-down layer is located between the separator andthe positive electrode, since the shut-down layer is for blocking thecurrent, and thus the shut-down layer may be located between theseparator and the positive electrode. In this case, even if a dendritecontinues to grow and penetrates the separator and reaches the shut-downlayer, the shut-down layer will melt, and thereby block the pores byheat generated in a defluoridation reaction of the dendrite (alkalimetal) and the separator (TFE). Thus, the dendrite can be certainlyprevented from penetrating the separator and causing a short circuitfrom the negative electrode to the positive electrode.

The present invention also relates to a separator used in the secondarybattery described above.

EXAMPLES

The present invention will now be described specifically by way ofexample, but the present invention is not limited by them.

In the Examples, various coin cells were produced under the conditionsshown in Table 1. These coin cells were used to perform acharge-discharge test (coin cell cycle by Li/Li), calculate the numberof cycles until an internal short-circuit due to a Li dendrite occurs,and evaluate the life of each coin cell. A description will now be givenin more detail.

TABLE 1 Covering Internal Material or Exposed Surface SeparatorLaminating Rate Coverage Material Material % % Example 1 PTFE SiOx 50 95Example 2 PTFE SiOx 70 95 Example 3 PTFE SiOx 90 95 Example 4 PTFE SiOx30 95 Example 5 PTFE SiOx 20 95 Example 6 PTFE PVDF 50 95 Example 7 PTFEPI 50 95 Example 8 PTFE PE laminate 50 — Example 9 PTFE PP laminate 50 —Comparative PE — — — Example 1 Comparative PP/PE/PP — — — Example 2Comparative Glass — — — Example 3 fiber cloth Comparative PTFE — 60 —Example 4

As a separator, a PTFE membrane (available from W. L. Gore & Associates,Inc.) was employed in Examples 1 to 9 and Comparative Example 4. InComparative Example 1 to Comparative Example 3, generally availableporous membranes shown in Table 1 were employed. In all the Examples,the membrane thickness, except for a glass fiber cloth was about 25 μm,and the porosity was near about 50%. The membrane thickness of the glassfiber cloth was 100 μm.

In Examples 1 to 9 and Comparative Example 4, silica was used tohydrophilize the separator. In particular, in Examples 1 to 5, the rateof hydrophilization was varied between 10% and 80% (20% to 90% in termsof internal exposed rate). One hundred parts of tetraethoxysilane(available from Shin-Etsu Silicone), 52 parts of water, and 133 parts ofethanol were allowed to react at 80° C. for 24 hours under reflux wherethe moisture from the outside air is blocked using a calcium chloridetube to prepare a partially gelled solution of metal oxide precursor.The PTFE membrane described above was impregnated with a diluent of thissolution, and then immersed in warm water at 60° C. to complete thegelation. This was dried in a thermostat bath at 150° C. for 30 minutesto obtain a separator whose exposed surface including the inner surfaceof a porous body was covered with silica gel and hydrophilized. The rateof hydrophilization was adjusted with the dilution rate of the partiallygelled solution of metal oxide precursor.

In Examples 1 to 9, as a layer that does not react with a dendrite, thecovering material or laminated material shown in Table 1 was providedbetween the separator and the negative electrode.

In Examples 1 to 5, the separator was coated with SiOx (glassysubstance) as a layer that does not react with a dendrite.

An SiOx coating agent (New Technology Creating Institute Co., Ltd.,SIRAGUSITAL B4373 (A), solid content: 60%) was dissolved in an IPAsolvent to adjust the solid content concentration of the SiOx coatingagent to 5%.

A porous PTFE film having a thickness of 25 μm was coated only on thesurface layer with the SiOx coating agent subjected to the concentrationadjustment described above by the gravure coating method.

For the dry conditions, after preliminary drying at 60° C. for 1 hr,curing was carried out in the environment at room temperature of 25° C.and 60% (relative humidity) for 96 hr.

The thickness of the layer that does not react with a dendrite was 0.2μm. The thickness was measured based on an observation of the thicknessof the SiOx layer on the surface of the PTFE membrane (separator) byusing a TEM (Transmission Electron Microscope).

In Example 6, PVDF (maker: ARKEMA, specification: KYNAR710) wasdissolved in a given organic solvent to a given concentration, and theresultant was applied and dried in the same manner as in Example 1.

In Example 7, PI (maker: Hitachi Chemical Co., Ltd., specification: HCl)dissolved in a given organic solvent was applied, dried, and cured inthe same manner as in Example 1.

In Example 8, as a layer that does not react with a dendrite, a PEporous membrane (membrane thickness: 25 μm, porosity: 50%) was laminatedon the separator.

In Example 9, as a layer that does not react with a dendrite, a PPporous membrane (membrane thickness: 25 μm, porosity: 50%) was laminatedon the separator.

For the hydrophilized separators in Examples 1 to 9 and ComparativeExample 4, to what extent the inner surface of pores was covered withsilica was measured. The results are expressed as Internal Exposed Ratein Table 1. Here, Internal Exposed Rate (%)=100−Coverage (%). Thecoverage (%) of silica corresponds to the rate of hydrophilizingtreatment. In Example 1, as shown in Table 1, 50% of the inner surfaceof pores was covered with silica, and the internal exposed rate was 50%.In measuring the coverage (internal exposed rate), X-ray PhotoelectronSpectrometer JPS-9200S for microanalysis manufactured by JEOL Ltd. wasused. The measurement conditions were as follows: filament current: 4.5A; and accelerating voltage: 4.0 eV. This apparatus was used toquantitatively determine the amount of F, O, C, and Si on the pore innersurface. In the case of the PTFE before being covered with silica,F/C=2:1 (66.7:33.3%). Based on this ratio, the coverage of silica wascalculated from the ratio of the surface F quantitated.

For the areas covered with the covering material of Examples 1 to 7(corresponding to the layer that does not react with a dendrite), towhat extent the inner surface of its pores was covered with the coveringmaterial was measured. The results were as shown in Table 1, and 95% ofthe inner surface of the pores was covered with the covering material.The measurement method was similar to the above.

<Production of Coin Cell>

As an electrode, two pieces of Li of φ 14 mm and a thickness of 100 μmwere provided (8.21 mg, 31.7 mAh). The separators and the layers that donot react with a dendrite in Examples 1 to 9 and Comparative Examples 1to 4 were formed into φ 17 mm. As an electrolyte solution, 1 moldm⁻³LiPF₆/EC:PC=1:1 was provided. These members were incorporated into a2032 coin cell available from Hohsen Corp. in a glove box to produce acoin cell of FIG. 1.

<Charge-Discharge Test>

This coin cell was used to perform a charge-discharge test (coin cellcycle by Li/Li). The charge and discharge measurement was carried outusing a battery charge and discharge apparatus (HJ1001SM8A) manufacturedby HOKUTO DENKO CORP. A charge-discharge test at a current density of 10mA/cm² for 30 minutes (DOD: depth of discharge, about 25%) was repeated.The number of cycles until an internal short-circuit occurs due to adendrite was calculated. The results are shown in Table 2.

TABLE 2 Cycle test result Covering The Number of Material or CyclesBefore Separator Laminating Short Circuit Material Material NumberExample 1 PTFE SiOx 1000 or more Example 2 PTFE SiOx 1000 or moreExample 3 PTFE SiOx 1000 or more Example 4 PTFE SiOx 1000 or moreExample 5 PTFE SiOx 1000 or more Example 6 PTFE PVDF 1000 or moreExample 7 PTFE PI 1000 or more Example 8 PTFE PE laminate 1000 or moreExample 9 PTFE PP laminate 1000 or more Comparative PE — 110 Example 1Comparative PP/PE/PP — 145 Example 2 Comparative Glass — 35 Example 3fiber cloth Comparative PTFE — 200 Example 4

As shown in Table 2, in every case of having a separator that reactswith a dendrite and a layer that does not react with a dendrite inExamples 1 to 9, the number of cycles before a short circuit is 1000 ormore. On the other hand, in the cases of not having a layer that doesnot react with a dendrite in Comparative Examples 1 to 4, the number ofcycles before a short circuit dramatically decreased.

It was confirmed that the secondary battery of the present invention isable to inhibit the growth of a dendrite that can generate from anelectrode comprising alkali metal.

The invention claimed is:
 1. A secondary battery, comprising: a positiveelectrode; a negative electrode comprising alkali metal; a separatorcomprising a layer of tetrafluoroethylene (TFE) polymer or copolymerthat reacts with a dendrite of the alkali metal, the separator isconfigured to be hydrophilized at a rate of not less than 10% and notmore than 80%; and a layer that does not react with a dendrite of thealkali metal located between the separator and the negative electrode.2. The secondary battery according to claim 1, wherein the layer thatdoes not react with a dendrite of the alkali metal is a part of theseparator, and, in the layer that does not react with a dendrite of thealkali metal, the inner surface of its pores is at least partiallycovered with a material that does not react with a dendrite of thealkali metal.
 3. The secondary battery according to claim 1, wherein thelayer that does not react with a dendrite of the alkali metal isindependent of the separator.
 4. The secondary battery according toclaim 3, wherein the layer that does not react with a dendrite of thealkali metal comprises any one of glass comprising SiO_(x) (0<x≦2),polyvinylidene fluoride (PVDF), polyimide (PI), polyethylene (PE), orpolypropylene (PP) or a mixture thereof.
 5. The secondary batteryaccording to claim 3, wherein the layer that does not react with adendrite of the alkali metal comprises any one of inorganic oxidesselected from the group consisting of alumina, titanium oxide, sodiumoxide, calcium oxide, boron oxide, potassium oxide, and lead oxide or amixture thereof and a binder.
 6. The secondary battery according toclaim 1, wherein in the layer that does not react with a dendrite of thealkali metal, the inner surface of its pores is covered with a materialother than tetrafluoroethylene (TFE) polymer or copolymer.
 7. Thesecondary battery according to claim 6, wherein the material other thantetrafluoroethylene (TFE) polymer or copolymer is any one of glasscomprising SiO_(x) (0<x≦2), polyvinylidene fluoride (PVDF), polyimide(PI), polyethylene (PE), or polypropylene (PP) or a mixture thereof. 8.The secondary battery according to claim 1, wherein the layer that doesnot react with a dendrite of the alkali metal is hydrophilized.
 9. Thesecondary battery according to claim 1, wherein the tetrafluoroethylene(TFE) polymer or copolymer is expanded or expanded porous.
 10. Thesecondary battery according to claim 1, wherein the tetrafluoroethylene(TFE) polymer or copolymer is expanded polytetrafluoroethylene,perfluoro alkoxy alkane (PFA), tetrafluoroethylene/hexafluoropropenecopolymer (FEP), ethylene/tetrafluoroethylene copolymer (ETFE), orethylene/chlorotrifluoroethylene copolymer (ECTFE) or a mixture thereof.11. The secondary battery according to claim 1, wherein the thickness ofthe layer that does not react with a dendrite of the alkali metal is 0.1μm or more.
 12. The secondary battery according to claim 1, wherein theseparator at least comprises fluorine that can react with the total massof the alkali metal constituting the negative electrode.
 13. Thesecondary battery according to claim 1, wherein the alkali metal islithium or sodium.
 14. The secondary battery according to claim 1,wherein the secondary battery further comprises a shut-down layer. 15.The secondary battery according to claim 14, wherein the shut-down layeris located between the separator and the positive electrode.
 16. Theseparator used in the secondary battery according to claim 1.